1. Field of the Invention
This invention relates to means for eliminating liquid flow stagnation on vapor-liquid contacting trays in the region of a support ring on which the tray is mounted.
2. Description of the Prior Art
In the general practice of vapor-liquid contacting, as for example in distillation and absorption applications, contacting columns are commonly employed which comprise a casing in which a plurality of horizontally aligned vapor-liquid contacting trays are disposed. These contacting trays are vertically spaced apart from one another and are mounted in the casing on imperforate support rings joined to an inner surface of the casing. The imperforate support rings extend inwardly from the casing such that the support rings underlie the peripheral portions of the contacting trays for support thereof. Typically, clamp means are employed to rigidly secure the contacting trays to the support rings.
The vapor-liquid contacting trays employed in the conventional contacting column are commonly of the well-known cross-flow type. Such contacting trays include a horizontally aligned deck having main flat top and bottom surfaces, with vapor flow perforation openings extending through the deck. Liquid inlet means are provided at one edge of the tray deck and liquid discharge means at an opposite edge thereof whereby a liquid flow path is formed extending across the tray deck from the inlet to the liquid dischage. Such trays may be of the single pass or multiple pass types with cross-flow or parallel flow of liquid on the tray deck.
In the use of the above-described vapor-liquid contacting systems, a serious hydraulic flow stagnation problem has been found to be associated with the peripheral portions of the vapor-liquid contacting tray. One reason for such flow stagnation problem is that the periphery or outer edge of the vapor-liquid contacting tray is bounded by an inner wall surface of the contacting column casing. Liquid flowing across the main flat top surface of the tray deck from the liquid inlet to the liquid discharge in the peripheral portions of the flow path adjacent the inner wall surface of the contacting column casing experiences a frictional drag force from the casing wall surface. Although a frictional drag force is imposed by the tray deck main flat top surface on the liquid flowing across the gas-liquid contacting surface, the overall frictional drag force exerted on the liquid in the region of the casing wall surface is substantially greater. Experience with cross-flow vapor-liquid contacting trays has demonstrated that the existence of frictional wall surface drag causes the liquid flow to be retarded near the contacting column wall while the liquid in the central portions of the tray move with a reasonably uniform velocity across the tray deck from the liquid inlet to the liquid discharge. The deficiency due to frictional wall drag is compounded by the lateral distribution of liquid residence times on the tray surface, arising from the fact that the liquid flow path adjacent the casing wall regions of the tray is longer than in the central region of the tray. Such phenomena result in the occurrence of liquid flow stagnation along the peripheral portions of the liquid flow path adjacent the inner surface of the contacting column casing.
The aforementioned liquid flow stagnation along the peripheral liquid flow path portions adjacent the inner surface of the contacting column casing is associated with severely reduced contacting efficiency for the overall tray. In the stagnant wall region of the vapor-liquid contacting surface, the liquid does not replenish itself at a sufficient rate as in the central portions of the tray surface. As a result, the liquid in the wall surface regions will rapidly reach equilibrium with the gas or vapor stream passing upwardly through the vapor flow perforation openings in the tray deck and little subsequent mass transfer will take place on that part of the tray. The peripheral liquid flow path portions of the tray adjacent the inner surface of the contacting column casing thus constitute tray areas in which relatively little mass transfer takes place, thereby contributing to a significant reduction in the overall tray contacting efficiency.
Another operating deficiency associated with the peripheral portions of the tray adjacent the inner surface of the contacting column casing is the low extent of vapor-liquid contacting on such portions of the tray due to the presence of the aforementioned support rings. As described, imperforate support rings are conventionally joined to an inner surface of the casing and extend horizontally inwardly therefrom such that the support rings underlie the peripheral portions of the contacting tray for support thereof in the contacting column. In commercial size contacting columns, the imperforate support rings normally extend inwardly a distance of two to four inches from the inner surface of the contacting column casing. Typical crossflow vapor-liquid contacting trays employ a multiplicity of vapor flow perforation openings distributed across and extending through the tray deck for flow of the generally upwardly flowing gas into the liquid supported on the main flat top surface of the contacting tray. When such contacting trays are installed in a contacting column, the circumferentially extending imperforate support rings effectively block any vapor flow openings in the peripheral portions of the tray which overlie the support ring in the portions of the tray adjacent to the inner surface of the column casing. The tray area adjacent the inner surface of the column casing therefore becomes inactive in operation, since vapor is prevented from contacting liquid on this portion of the tray surface. The consequences of the resultant less than full utilization of the column cross-section for vapor flow include an increase in vapor velocities through the vapor flow perforation openings on the remainder of the tray deck, a decrease in local mass-transfer efficiency in the central portions of the tray due to higher superficial velocity of the vapor through the perforation openings in the tray deck and a resultant tendency toward entrainment, and reduced flooding limits for the tray.
Accordingly, it is an object of the present invention to provide means for eliminating liquid flow stagnation on vapor-liquid contacting trays in the region of the inner surface of the casing in which the tray is mounted.
In addition, it is an object of the present invention to provide means for insuring vapor-liquid contact in the peripheral liquid flow path portions of the tray deck adjacent the inner surface of the contacting column casing, where vapor-liquid contact is normally prevented by the circumferentially extending imperforate support ring.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.